This invention relates to devices and methods for chemical analysis of materials. Specifically this invention relates to headspace analysis, and a headspace instrument used to increase sensitivity and repeatability in conducting headspace analysis.
Headspace analysis is a relatively new technique which enables the sampling of a vapor phase of a sample with an analytical instrument. Headspace sampling ensures that only volatile materials are delivered for analysis. For example when the analytical instrument used is a gas chromatograph, headspace sampling assures that only volatile species that can be eluted from a column of a gas chromatograph will be introduced into the instrument.
In headspace sampling a volatile non-vapor phase of a substance being analyzed, which may be either liquid or solid, attains equilibrium with a vapor phase of the substance within a sealed vial. Equilibrium is established when the non-vapor phase of the substance in the vial no longer changes so that the total quantity of the vapor and non-vapor phases remains constant. Often a syringe is used to retrieve a small sample of the vapor for analysis. The retrieved vapor is then introduced into an analytical instrument. Headspace technology is advantageous over conventional direct sample injection techniques because it allows only vapor to enter the analytical instrument. This is advantageous because it reduces the chance of contamination or damage to the analytical instrument due to introduction of unevaporated sample material. Because the sample is in vapor form, greater sample volumes may be supplied to the instrument. Increased sample size generally results in increased sensitivity.
Samples of headspace vapor may be extracted from a sample vial using a number of other techniques. Such techniques often involve equilibrating the vapor and non-vapor phase of a substance for analysis within a closed vial. A sample needle is moved to pierce a septum bounding the headspace in the vial. As a result a fluid passage through the needle is in fluid communication with the vapor phase of the sample in the headspace. To extract the headspace sample it is usually necessary to first pressurize the headspace with a suitable gas.
After the headspace has been pressurized, the pressure is released allowing the sample material to pass out of the vial and into an analytical instrument or other device for collecting or analyzing the sample. Techniques for extracting headspace vapor from a vial are shown in U.S. Pat. No. 5,441,700 the disclosure of which is incorporated by reference as if fully rewritten herein.
A drawback associated with conventional techniques for the extraction of sample vapor from a headspace vial is that variations in pressure must be achieved to extract the sample material. Such variations in pressure often change the equilibrium conditions between the vapor phase and the non-vapor phase of the substance being analyzed. Changes in equilibrium may change the makeup of the headspace vapor. Such changes which result from the sampling process often impact the results in ways that are undesirable.
Thus there exists a need for a headspace instrument and method of operation which minimizes the effects of the sampling process on the constituents in the sample and which increases sample volumes which may be delivered and/or analyzed by an analytical instrument. There further exists a need for a headspace instrument which may be used to carry out analysis on time or concentration dependent materials.
It is an object of the present invention to provide a headspace instrument which achieves the sampling of headspace vapors while minimizing the disturbance of thermodynamic equilibrium.
It is a further object of the present invention to provide a headspace instrument which achieves improved sensitivity.
It is a further object of the present invention to provide a headspace instrument which increases sample volumes which may be analyzed by an analytical instrument.
It is a further object of the present invention to provide a headspace instrument which implements improved sampling methods.
It is a further object of the present invention to provide a headspace instrument which obtains equilibrated samples of headspace vapors under conditions of thermodynamic equilibrium.
It is a further object of the present invention to provide a headspace instrument which reduces the possibility for cross contamination between samples.
It is a further object of the present invention to provide a headspace instrument that may be used to measure time dependent or concentration dependent processes in analyzed samples.
It is a further object of the present invention to provide an improved method of sampling materials for analysis.
It is a further object of the present invention to provide a method of sampling that reduces the effects of pressure changes on a headspace sample.
It is a further object of the present invention to provide a method of sampling that may achieve greater sample size.
It is a further object of the present invention to provide a method of sampling which achieves increased sensitivity.
It is a further object of the present invention to provide a method of sampling under conditions of thermodynamic equilibrium.
Further objects of the present invention will be made apparent in the following Best Modes for Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in an exemplary embodiment of the present invention by headspace instrument which handles a vial holding a vapor and a non-vapor phase of a substance for analysis. A first sample needle extends into the headspace in the vial which holds a vapor phase of the substance. The first needle is in fluid connection with a first variable volume chamber. A second sample needle extends in the vapor phase of the substance in the headspace of the vial as well. The second sample needle is in fluid communication with a sample loop and a second variable volume chamber. The volumes of the first and second variable volume chambers are varied in a periodic and controller manner. The volumes of the first and second variable volume chambers while varied in a periodic manner, maintain the total volume of the two chambers as a constant volume. This causes the vapor phase of the substance to flow between the headspace of the sample vial, the sample loop and the variable volume chambers. As a result a sample of the equilibrated vapor phase of the substance passes through the sample loop without disturbance of the thermodynamic equilibrium between the vapor phase and the non-vapor phase of the substance. The aliquot of material in the sample loop is then analyzed using a gas chromatograph or other analytical instrument.
Alternative embodiments of the invention may be used to analyze samples which undergo time dependent or concentration dependent processes. Alternative embodiments may also be used to change the concentration of analytes or to add analytes to the material subject to analysis. Embodiments of the invention may also be used to carry out adsorption or desorption experiments. Exemplary embodiments of the invention may also include provisions for purging sample vapors from the systems to minimize the risk of cross contamination between samples.